Constant time insert into linked list of resources.

Constant time insert into linked list of resources.

Do not iterate over linked list to insert at end.
We already iterate once to check if resource uri already exists.
Use a pointer to the tail of the linked list to insert resource.
Changed two extern variables to static as they're only used in one module.

Change-Id: I41f458a2384b4fa2d8a23d78e632e1614d4875c8
Signed-off-by: Mandeep Shetty <mandeep.shetty@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/365
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Doug Hudson <douglas.hudson@intel.com>
Reviewed-by: Erich Keane <erich.keane@intel.com>